Fuel cylinders for liquefied natural gas (LNG), liquefied petroleum gas (LPG), and particularly for hydrogen gas (H2) are ideally as light as possible. Cylinder structures used to maintain the high pressures and remain lightweight include use of aluminum cylinders or liners wound with carbon fibres and plastic liners or bladders similarly wrapped in carbon fibres. The carbon fibre wraps provide the necessary structural integrity where less structurally-capable, yet low permeability, fuel-retaining liners are used.
Cylinder structures capable of containing high pressures utilize hemispherical or polar heads. Whether lightweight metal cylinders or plastic liners are used, one or more outlets are presently formed of metal. For example, a polar head of a fibre-wrapped, plastic lined vessel can be fit with a metal boss. Usually, the metal boss is integrated with the liner prior to wrapping with carbon fibres. The boss can have a partial polar shoulder which is over-wrapped as well with the carbon fibres. The boss is threaded and fit with an “end fitting” or insert—including a plug, valve or pressure regulator.
The boss itself is needed for installation of an end fitting or insert such as a valve or regulator. It is known that the conventional threaded connections between boss and insert are not adequate to block a leak path of the pressurized fluid within, and particularly challenging with low molecular weight gases.
Current versions of lightweight fuel cylinders typically comprise a plastic liner fit with a metal boss and then wrapped with carbon fibre. Due to the introduction of plastic liners, there is now a new problem introduced with sealing of the boss at the entrance to the cylinder. There is a problematic interface between the liner and the polar end ports used to access the interior of the cylinder. The interface of the plastic liner and the metal boss has the potential to leak.
Some manufacturers go to great lengths of providing a seal interface between the boss and the liner by leaving the bore of the boss free of any liner components and accepting conventional fittings. An example is a tongue and groove form of interface as set forth in U.S. Pat. No. 6,227,402 to Shimojima et al. wherein the plastic liner is integrated into annular grooves in the boss. Other forms where a boss embedded in the liner itself are set forth in U.S. Pat. No. 5,253,778 to Sirosh, U.S. Pat. No. 5,518,141 to Newhouse et al. and U.S. Pat. No. 7,549,555 to Suzuki et al. When a leak does occur, the cylinder cannot be repaired and is scrap.
A characteristic of plastic is a higher rate of creep under sustained loading. In this case, creep is exhibited at the seal interface of the liner and the insert. Seals typically comprise an elastomeric seal element compressed against rigid seat. With the introduction of plastic liners, the rigid seat is replaced with the plastic material. Over time, plastic tends to slowly move away from a sealing engagement with the seal element and pressurized fluid then can escape thereby.
It is known in the prior art to introduce a liner outlet into the bore of the boss. Accordingly, there has been an attempt to provide a seal between the plastic liner within the boss and the insert. In U.S. Pat. No. 5,938,209 to Sirosh et al. and U.S. Pat. No. 6,186,356 to Berkley et al., an O-ring is sandwiched axially between an annular end face of the liner and end face of the insert. In another form, as set forth in published US Application 2009/0071930 to Sato et al., an O-ring is located between the liner and the boss. Again, should a leak occur, the cylinder cannot be repaired and is scrap.
Other prior art arrangements include placing an O-ring circumferentially in an annular groove formed in the insert, the insert and O-ring portion protruding into and sealing against a cylindrical throat of the liner.
Other factors contributing to seal leakage include differential thermal expansion of the differing materials. The insert is usually aluminum or stainless steel which has a lower coefficient of thermal expansion than plastic which can also cause issues at the interface.
Accordingly, a new sealing system would overcome the deficiencies experienced by the prior art.